Neutron scattering: a valuable tool for the advancement of sustainable polymers
Polymers – macromolecules composed of repeating subunits called monomers – are everywhere. Proteins, DNA, starch and cellulose are all examples of naturally occurring polymers, while synthetic polymer materials – considered one of the most important inventions of modern times – are involved in most widely used and indispensable materials such as plastics, rubbers, concrete, glass and paper.
Despite the large variety of polymers, synthetic polymer production is dominated by plastics, leading to the generation of about 400 million tonnes of plastic waste a year, of which less than 10% is currently recycled. The development of sustainable plastics, with equivalent properties to traditional petroleum-based polymers while remaining cost-competitive, is thus a research area of critical societal and environmental importance. The value of neutron scattering for research in the domain has been recently highlighted by experiments carried out at the Institut Laue Langevin (ILL) on polymers synthesised at the University of Bath.
PLA (polylactide) is the most produced bio-based polymer today and one of the first sustainable plastics capable of competing with conventional polymers. Made using renewable raw materials, the production of PLA generates three times less CO2 than typical polymers and is both recyclable and compostable at end-of-life, with biodegradation taking months compared to the hundreds or thousands of years required by fossil-based plastics.
Though used in packaging, bottles and biomedical applications, the quantity of PLA produced (around 200,000 tonnes per year) is minor in comparison to the millions of tonnes of plastics made from petrochemicals. “The reason why we don’t see bio-based and biodegradable polymers in huge quantities at the moment is that the versatility of their properties – what the polymer can do and where it can be used – can’t yet compete with typical petroleum-derived polymers,” explains Philip Yang, PhD student at the Institute for Sustainability at the University of Bath.
Recent progress in polymer synthesis has enabled the production of pure, well-defined cyclic polymers in large quantities. The cyclic topology – where the usual linear chain of monomer units forms instead a circular structure with the chain ends joined – is responsible for a range of unique physical and chemical properties: lower viscosities, higher transition temperatures, better thermal stability and faster crystallization rates compared to conventional linear polymers. The cyclic form of PLA is thus attracting significant interest through its potential to improve the range of properties and thus expand the versatility of PLA.
Detailed studies of cyclic polymers are now necessary in order to better understand and advance the development of these polymers. “The high sensitivity of neutrons to hydrogen in combination with the powerful selective deuteration contrast method makes neutrons a particularly valuable probe for studying these hydrogen-based polymers, capable of providing unique and precise information,” explains Olga Matsarskaia, scientist and co-responsible for the D11 small-angle neutron scattering (SANS) instrument at the ILL.
Samples of linear poly(lactide) and cyclic poly(rac-lactide) were synthesised at the University of Bath and transferred to the ILL for SANS experiments using the D11 instrument. “The particularly high neutron flux at the ILL provides an excellent signal-to-noise ratio for even weakly scattering systems as well as shorter counting times per sample,” explains Matsarskaia. “This high efficiency means that a number of different experiments can be performed during the allocated beamtime, enabling a detailed study to be carried out which is necessary to achieve an in-depth understanding of the system.” For this experiment, scattered neutron intensities were collected for a large array of samples while varying a range of different parameters: solvents (acetone-d6 and THF-d8), temperatures (15 and 40°C), molecular weights, microstructures and sample-to-detector distance to cover a range of q (scattering variable).
The acquired data, reduced by Matsarskaia at the ILL and analysed at the University of Bath, enabled a comparison of the two PLA sample topologies (linear and cyclic) across a range of variables. The results were also compared with a previously reported SANS investigation of linear and cyclic polystyrene samples. “Polystyrene is probably the most commonly reported cyclic polymer. As a petroleum-derived polymer, however, it has a very different chemical structure to bio-based PLA,” explains Yang. The findings, published in Macromolecules in 2022, followed previously established trends but with much more substantial variations between the linear and cyclic topologies than previously reported. “In our study, we observed much greater differences between the linear and cyclic forms of PLA than the authors of the previous study reported for polystyrene. The results thus show that not all cyclic polymers will behave like linear polymers,” explains Yang.
The results highlight the value of neutron scattering as a tool for the identification and characterisation of the cyclic topology and for the comparison of linear and cyclic polymers. Further neutron scattering experiments, this time employing the technique of quasielastic neutron scattering (QENS), are planned at the ISIS Neutron and Muon Source (UK) to compare linear and cyclic PLA in bulk form rather than dissolved in solvent. The information provided by these neutron scattering studies improves the understanding of cyclic polymers, necessary in order to guide the development and advance the commercialisation of sustainable polymers.